More than half of all inmates in the Hennepin County jail suffer from a mental illness of some kind — far more than previously thought — and they are more likely to reoffend than other inmates, according to a detailed new jail study.

The findings, based on a one-day survey of 680 inmates, are likely to buttress ambitious new efforts by the county to break the pattern of offenders with mental health problems cycling in and out of courts and jails without receiving adequate treatment.

“What we’re seeing is crisis levels of mental illness among our inmates,” said Hennepin County Sheriff Rich Stanek. “This is solid evidence that our jails continue to serve as the largest mental health facilities in the state.”

Next month, Hennepin County will launch a new program designed to better identify inmates with mental health problems and divert many of them to court-supervised treatment programs. For the first time, doctoral-level psychologists will work at the jail to evaluate inmates and identify treatment alternatives in the community. The program is expected to result in the early release of at least 100 mentally ill inmates in the first year, but only those accused of lower-level offenses.

In addition, jail officials have begun to collaborate more closely with Hennepin County Medical Center (HCMC). Starting next month, every inmate booked into the downtown jail will be screened for a mental illness by a registered nurse from HCMC; previously, such screening was done by sheriff’s deputies and was less effective. Inmates who require prescription medications will receive a 24 hours’ supply upon release, with HCMC nurses overseeing the service, Stanek said.

The recent one-day survey found that 11 percent of the jail population receive antipsychotic medications, for conditions such as schizophrenia and bipolar disorder. “It speaks to the level of acuity of these individuals and how much they’ve suffered,” said HCMC psychiatrist Dr. Ian Heath.

Jail officials say that each day, dozens of people with psychiatric problems are brought into custody at the downtown Minneapolis complex for minor crimes, such as disturbing the peace or public urination. Most are released within a few hours or days, but many languish for weeks with little psychiatric care and often leave jail in a weaker mental state than when they entered and go on to commit worse offenses. “We manage the issues they present, but we don’t treat them,” Stanek said.

The problem is not confined to Hennepin County. In a report issued early this year, state Legislative Auditor James Nobles found that jails across Minnesota had inadequate staffing and skills to handle the large population of inmates with mental health needs. Since 2000, there have been more than 50 suicides and 770 suicide attempts in Minnesota jails — some preventable, Nobles found.

Even so, the recent analysis by the Hennepin County Sheriff’s Office suggests the problem is much greater than previously thought. For years, county officials have estimated that about 25 to 30 percent of the overall inmate population had a mental illness, based on screenings done by sheriff’s deputies. The more in-depth analysis, conducted on one day in July, found that 354 individuals, or 52 percent of the total inmate population, showed indicators of a mental illness. This includes 243 individuals who had a diagnosed mental illness or had prescriptions for antipsychotic medications; and another 111 individuals who displayed behaviors associated with a serious mental illness or had a “significant history connected to mental illness.”

While the severity of their offenses was not significantly different from the general population, inmates with mental health disorders were far more likely to have had repeat bookings in the Hennepin County jail, the county found. Inmates with indicators of mental illness were 30 percent more likely to have had 10 or more bookings in the Hennepin County jail than those who showed no signs of mental health problems.